<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
      div {
          margin: 3.1em auto;
          background: #fb3;
          width: 200px;
          height: 100px;

          /*椭圆*/
          /*border-radius: 50% / 50%;*/

          /*半椭圆*/
          /*border-radius: 50% / 100% 100% 0 0;*/

          /*四分之一椭圆*/
          /*border-radius: 100% 0 0 100% / 50%;*/

          border-radius: 100% 0 0 0;
      }
    </style>
</head>
<body>
    <div>
        
    </div>
</body>
</html>